Luminaires that rely upon prismatic reflectors of the above type are conventional and are disclosed in the following exemplary U.S. Pat. Nos. 2,818,500--Franck; 1,259, 493--Dorey; and 1,758,977--Rolph. These prismatic reflectors rely upon the principle of total internal reflection of visible light at a glass-to-air interface when the light is incident upon that surface at an angle greater than the critical angle of incidence, which for a glass-to-air interface is typically about 42.degree.. Typically, each prism has two reflecting surfaces, one of which reflects light onto the other, and each being oriented to provide an incidence angle of greater than the critical angle with respect to the light incident thereon. The reflecting surfaces of each of these prisms meet at an apex, and the adjacent reflecting surfaces of juxtaposed prisms meet in a valley between the prisms at the nadir of the valley.
A disadvantage of this type of reflector is that if a non-uniform film is allowed to deposit on the prismatic surface, the desired reflecting properties of the surface may be significantly degraded, especially if the film is partially or fully light-absorbing. If the deposit is not uniform, it effectively changes the incidence angle of light rays striking the relevant interface of the prism with the adjacent air and may let light through this first interface or may change the angle of reflection so much at the first interface that the incidence angle at the second interface is less than the required critical angle, thereby letting the light pass through at the second interface.
Another characteristic of prismatic reflectors of the above type is that a substantial amount of light will pass through the reflector even though the reflecting surfaces of the prisms are clean. One reason for this is that the molds typically used for making the prismatic reflectors are not precise enough to achieve mathematical precision of the reflecting surfaces all the way to the apices of the prisms and to the nadir of the valleys between them; and, consequently, light leakage will occur in these regions. Additional light leakage can occur at points of defects in the prism surfaces, which points may be present as a result of mold imperfections or even designed-in defects. Altogether, this light leakage can typically amount to about 25% of the lumen output of the luminaire. In certain luminaire applications, specifically, those with bare reflectors, this light leakage has been advantageously employed to provide uplight, so as to reduce ceiling contrast and to illuminate overhead structures.
To protect the optical properties of the prismatic surface of the reflector from being degraded by the deposition thereon of film or other contaminants, the reflector has sometimes been provided with a form-fitting metal cover, surrounding the reflector and spaced a small distance from its outer surface. A disadvantage of this construction is that the metal cover defeats the often-desired uplighting abilities of the luminaire.